Alcatraz Island () is a small island in San Francisco Bay, 1.25 miles (2.01 km) offshore from San Francisco, California, United States. The island was developed in the mid-19th century with facilities for a lighthouse, a military fortification, and a military prison. In 1934, the island was converted into a federal prison, Alcatraz Federal Penitentiary. The strong currents around the island and cold water temperatures made escape all but impossible, and the prison became one of the most notorious in American history. The prison closed in 1963, and the island is now a major tourist attraction.

Beginning in November 1969, the island was occupied for more than 19 months by a group of Native Americans, initially primarily from San Francisco, who were later joined by AIM and other urban Indians from other parts of the country, who were part of a wave of Native American activists organizing public protests across the US through the 1970s. In 1972, Alcatraz was transferred to the Department of Interior to become part of Golden Gate National Recreation Area. It was designated as a National Historic Landmark in 1986.

Today, the island's facilities are managed by the National Park Service as part of Golden Gate National Recreation Area. Visitors can reach the island in fewer than 15 minutes by ferry ride from Pier 33, located between the San Francisco Ferry Building and Fisherman's Wharf, San Francisco. Hornblower Cruises, operating under the name Alcatraz Cruises, is the official ferry provider to and from the island.

Alcatraz Island is the site of the abandoned federal prison, the oldest operating lighthouse on the West Coast of the United States, early military fortifications, and natural features such as rock pools and a seabird colony (mostly western gulls, cormorants, and egrets). According to a 1971 documentary on the history of Alcatraz, the island measures 1,675 feet (511 m) by 590 feet (180 m) and is 135 feet (41 m) at highest point during mean tide. The total area of the island is reported to be 22 acres (8.9 ha).Landmarks on the island include the Main Cellhouse, Dining Hall, Lighthouse, the ruins of the Warden's House and Social Hall, Parade Grounds, Building 64, Water Tower, New Industries Building, Model Industries Building, and the Recreation Yard.